For gunite ( decorative concrete) swimming pools, the hole does not define the shape of the swimming pool, the gunite defines the shape. Decorative concrete swimming pools are an excellent choice because you can define the color of the swimming pool. The pool builders all have a wide selection of decorative concrete colors to pick from so you can decide the look of your swimming pool. After the hole for the swimming pool has been dug, the contractor will lay the plumbing for the pipes for drainage and skimmers. The contractor will then need to lay rebar in the grid pattern for the floor of the swimming pool and up the walls. The contractor will tie the rebar together using wire at the point of intersection of the rebar. This will create the shell for the shape of your swimming pool.